The present invention relates to a thermal energy storage apparatus adapted to transfer thermal energy to or from a moving air stream.
Thermal energy storage systems have heretofore been proposed as a means for shifting power consumption from peak demand periods to off peak periods. For example, it has been proposed to incorporate a cool storage medium in an air conditioning system, with the medium being cooled during off peak hours and then utilized to cool a building during peak hours. The most common storage medium has been water or ice. However, the use of water as a storage medium is not usually practical, since water has a relatively low freezing temperature, and thus the cooling unit must operate below freezing temperature to utilize the heat capacity of the water/ice transition.
It has also been proposed to use gas hydrates as a cool storage medium in an air conditioning system, note for example U.S. Pat. No. 4,540,501 to Ternes et al. Gas hydrates are a class of compounds described as non-stochiometric crystalline solids that are classed as clathrate compounds. More particularly, gas hydrates are solid crystalline structures, with the gas molecules trapped within the ice-type lattice. The trapped gas lends stability to the structure, which permits most such hydrates, which are principally water, to exist as a solid at temperatures well above the 32.degree. F. freezing point of water. The melting or decomposition of such gas hydrates in a sealed tube requires a heat input of approximately 120 Btu/pound. Gas hydrates therefore have cool storage capacity close to that of the ice/water transition (144 Btu/lb), but this capacity is deliverable at higher and more energy efficient temperatures. Thus, gas hydrates have an advantage as compared to pure water when used as a storage medium, in that the transition temperature is well above 32.degree. F., and the transition temperature can lie within the operating temperatures of conventional air conditioning systems.
The use of gas hydrates as a thermal energy storage medium thus has several recognized advantages as noted above. However its actual use has been discouraged by the fact that while the hydrate spontaneously decomposes at its transition temperature, the initial formation of the hydrate requires temperatures much lower than its transition temperature, particularly when all of the hydrate has been decomposed and none remains in the medium.
It is accordingly an object of the present invention to provide a thermal energy storage apparatus which effectively overcomes the above noted limitations and disadvantages of the known systems.
It is a more particular object of the present invention to provide a gas hydrate thermal energy storage apparatus which can be easily adapted for operation with conventional air conditioning or heat pump systems, and which is adapted to function either as a cool storage system or as a low temperature heat storage system.
It is still another object of the present invention to provide a gas hydrate thermal energy storage apparatus which has provision for encouraging hydrate formation at temperatures at or only slightly below the transition temperature.